Flue gas desulfurization (FGD) processes currently in use today typically employ wet calcium-based technology to remove sulfur from various flue gas sources Sulfur is absorbed from the flue gas as S0.sub.2 into a calcium-containing liquid phase and forms calcium sulfite or calcium bisulfite. At least some of calcium sulfite or bisulfite will be oxidized to calcium sulfate. The amount of calcium sulfate oxidized will depend, in large measure, on the amount of excess oxygen in the flue gas. Crystals of calcium sulfite or calcium sulfate, primarily as gypsum (CaS0.sub.4.2H.sub.2 O), will form as the respective critical relative saturations for each of the compounds is exceeded in the liquid phase. If the ratio of the amount of sulfite oxidized to sulfate compared to the total amount of sulfur compounds absorbed from the flue gas is less than 15 to 18%, all of the sulfur will be purged from the FGD system as a coprecipitate within the calcium sulfite crystal matrix. If this occurs, the calcium sulfate or gypsum relative saturation in the liquid phase will never exceed 1.0, and, therefore, calcium sulfate or gypsum scaling cannot occur.
The widespread use of calcium-based wet scrubbers for controlling SO.sub.2 emissions from utility boilers and the like generates as waste quantities of calcium sulfate and/or calcium sulfite solids in the range of 5 to 10 million tons annually in the United States. This material is currently disposed of primarily in ponds and landfills. The volume of this FGD waste material and the disposal methods currently employed depends largely on the chemical composition of the precipitated solids. Calcium sulfate dihydrate (gypsum) solids are generally larger and more regularly shaped than are calcium sulfite hemihydrate solids. While gypsum can usually be dewatered to produce about 85% solids, only about 50 to 70% calcium sulfite hemihydrate solids can typically be produced. Moreover, the calcium sulfite sludges tend to exhibit thixotropic-like behavior, which increases the difficulty and expense of their disposal.
The size and shape of gypsum crystals produced in the wet calcium-based FGD processes can affect the dewatering and handling characteristics of the resulting waste product. The performance of the dewatering equipment can be affected significantly by the size and shape of these gypsum crystals. If the gypsum particles are smaller than about 5 microns, they will impede filtration by blinding the filter media or forming a fine film on the solids being filtered, thus reducing the filtration rate and increasing the amount of moisture retained by the gypsum. Additionally, if the gypsum produced by FGD processes is to be a useful product, the ability to control the particle size, moisture content and impurity level is critical.
The use of FGD process-generated gypsum has been proposed for gypsum-based building materials such as wallboard. However, the inconsistent chemical purity and the lack of control of gypsum crystal size characteristic of gypsum byproducts of currently used FGD processes has often resulted in an inferior product. Impurities such as chloride in the gypsum result from adherent scrubber solution, and the amount of these impurities in the byproduct is directly related to solution composition and the degree of dewatering.
The gypsum disposal and use problems could be reduced by modifying available FGD processes to produce larger crystals. FGD process sludges could be dewatered more easily and with greater efficiency to produce a product with potential commercial utility if the size and shape of the crystals and particles were larger and more regular In addition, new systems could be designed with smaller and less expensive dewatering and disposal equipment.
The prior art has proposed solutions to the gypsum scaling problem in flue gas desulfurization systems. In these cases, additives have been used to prevent gypsum precipitation rather than modify the size and shape of the precipitated crystals. For example, U.S. Pat. No. 4,818,506 to Lin et al. discloses compounds, including organo-phosphonates, useful as gypsum scale inhibitors in flue gas desulfurization processes. These compounds affect the liquid phase chemistry to permit the toleration of a higher gypsum relative saturation without the occurrence of gypsum precipitation or scaling. The compounds disclosed by Lin et al. inhibit the formation of gypsum crystals until a higher liquid phase gypsum relative saturation exists. If the natural oxidation is not high enough, the higher liquid phase gypsum relative saturation may never be reached, and scaling will not occur. The solids produced by the system described by Lin et al. are a mixture of calcium sulfite and calcium sulfate mixed crystals. Increasing the size and regularity of the crystals is not suggested Moreover, the Lin et al. system specifically minimizes crystal growth. As a result, disposal of the Lin et al. byproduct solids remains a problem, and the reuse potential of these solids is extremely limited
U.S. Pat. Nos. 4,503,020 to Weissert et al.; 4,687,649 to Kuroda et al.; 4,810,477 to Shinoda et al. and 4,832,936 to Holter et al. all describe flue gas desulfurization processes which produce gypsum. 0f these processes, only that described in U.S. Pat. No. 4,503,020 specifically discloses a method for producing gypsum crystals with a particle size large enough to render the gypsum useful for the production of gypsum building materials such as plasterboard. This method processes the flue gas scrubber sump product with a thickener and circulates it for a rather lengthy period of time at a controlled temperature to produce calcium sulfate dihydrate (gypsum) in large crystal form. The crystals are dewatered and treated with sulfuric acid and steam to convert them to the alpha hemihydrate, which is free from sulfite. Although this method may be used to produce large crystals of gypsum, it provides no control over the size and shape of the crystals and, thus, the quality of the gypsum product. None of the other patents, moreover, suggests controlling FGD processes to control the production of gypsum crystals so that they have a size and shape that renders them useful for building materials or other gypsum products.
The prior art, therefore, has failed to provide a method of treating wet calcium-based flue gas desulfurization process byproduct solids which controls the size and shape of the gypsum crystals in these solids to permit the production of a commercially useful gypsum product. A need exists, therefore, for a method of treating wet calcium-based FGD byproduct solids under forced oxidation conditions which permits control of the size and shape of the crystalline solids.